1. Field of the Invention
This invention relates to the recording art and more particularly, to high density information records, such as video or digital audio discs, on which information signals are recorded as geometric variations.
2. Description of the Prior Art
In certain information playback systems, information signals are encoded as geometric variations or pits in a spiral plane or groove, or in concentric planes or grooves on an information recording medium or substrate. The medium on which the information signals have been recorded is reproduced in the following manner. When a pickup stylus such as a diamond having an electrode thereon is scanned relative to a recorded track of the medium, capacitive variations are established between the pickup stylus and the medium due to the geometric variations in the plane or groove.
Several types of information records have been proposed for use in the above-described type of electrostatic capacitance playback system. Typical of such an information record is one which is obtained by press molding a conductive plastic composition comprising a resin such as polyvinyl chloride and a conductive material such as carbon black.
When the press molding of vinyl chloride resin compositions comprising carbon black particles dispersed therein is repeated using stampers for mass production, fine defects are inevitably formed on the stampers. Once formed, the defects are invariably press molded, as they are, along with information signals. We have found that such defects cause signal defects when the defect-bearing record is played back. In other words, information signals to be recorded are not accurately formed on recording media owing to the presence of fine defects on the stampers.
In order to overcome the drawback caused by fine defects on stampers, we have made studies and, as a result, found that the fine defects are formed largely due to presence of impurities in carbon black. Carbon black ordinarily contain alkali metals and alkaline earth metals. Aside from these metals, silica, sulfur, aluminium and the like are also contained as impurities. Of these, silica and aluminium tend to damage the surfaces of stampers when a resin composition comprising carbon black is press molded under high compression pressure into video or digital audio discs. Sulfur serves to promote thermal decomposition of vinyl chloride resin and corrode processing machines. By the corrosion, foreign matters based on iron may be incorporated in the discs.
Moreover, when information records are placed under high pressure temperature and high conditions, calcium or alkali or alkaline earth metals derived from carbon black may cause rugged defects, leading to generation of signal defects upon playback of the records.